This proposal outlines the development of novel methods for the mapping of complex neural pathways and the specification of the neurons that compose these pathways. Our groups have recently made a recombinant Pseudorabies virus (PRV) that is conditional on cre-recombinase for viability and expression of a GFP reporter gene. This virus has been used successfully to map inputs to specific neuronal populations in defined brain regions. For simplicity we will refer to this and related methods as PAM, Pseudorabies virus assisted mapping. In this application, we propose to further develop PAM so as to allow: 1) Simultaneous Retrograde Tracing from Different Neurons in the Same Brain Region, 2) Simultaneous Retrograde Tracing from Similar Neurons in Different Brain Regions, 3) Anterograde tracing 4) Identification of Neurons Projecting to More than One Site 5) Specification of the neurons that are identified in these mapping studies. To establish the validity of the approach, these studies will be performed in analyses of hypothalamic circuits that regulate feeding. Once the methods are validated, the viruses and animals will be made freely available to other investigators interested in analogous studies of other neuronal pathways.